Microlenses are in widespread use in conjunction with light imaging arrays, particularly when these utilize photodiodes operating together with charge coupled devices (CCDs). The purpose of the microlens is to focus a pixel's worth of light onto the photodiode, thereby increasing the sensitivity of the display.
The easiest, and most commonly used, way to form a microlens has been to first form a disk of resin (which in practice is also a photoresist) and then to heat it until it softens, allowing surface tension forces to give it a hemisperical shape which approximates a plano-convex lens.
This process, while easy to use, is often difficult to control, making the future optical behavior of the microlens difficult to predict. Additionally, once the microlens has been formed the array must not be subjected to temperatures in excess of about 200.degree. C. or the microlens will melt. Such temperatures could be of external origin or they could be the result of lens heating. Finally, the optical transparency of photoresist tends to decrease over time, particularly when it is repeatedly exposed to light. Aside from decreasing the overall sensitivity of the array, this also serves to exacerbate the lens heating problem mentioned above.
The present invention provides a method of fabricating a microlens from silicon oxide, which material is much more robust than photoresist with respect to both temperature and light. In the course of searching for prior art we did not find that this method has been previously described. The following references were, however, found to be of interest:
Use of photoresist to form microlenses is discussed in detail by Sakakibara et al. in `A 1" format 1.5M pixel IT-CCD image sensor for a HDTV camera system` published in the IEEE Transactions on Consumer Electronics vol. 37 no. 3 August 1991, pp. 487-493. A similar discussion may be found in `Submicron spaced lens array process technology for a high photosensitivity CCD image sensor` by Sano et al. in IEDM 1990 pp. 283-286.
Furukawa et al. discuss the design of microlenses and the associated system in `A 1/3 inch 380K pixel IT-CCD image sensor` in the IEEE Transactions on Consumer Electronics vol. 38 no. 3 August 1992, pp. 595-600. The design of microlenses is also discussed by Deguchi et al. in `Microlens design using simulation program for CCD image sensor` in IEEE Transactions on Consumer Electronics vol. 38 no. 3 August 1992 pp. 583-589.
A non photoresist microlens, formed by reflowing a disk of boro-phosphosilicate glass (BPSG) at high temperature is described by Tsukamato et al. in `High sensitivity pixel technology for a 1/4 inch PAL 430K pixel IT-CCD` published in the 1996 Custom IC Conference, pp. 39-42.
We note here that all the above references describe processes that result in plano-convex lenses. Additionally, these processes do not, in general, provide for control of the distance between the microlens and the photodiode as an independent variable. Rather, this distance is determined by other parameters of the total process which must be compromised if said distance needs to be modified.